CAMBRIDGE, Mass. — Last year, eggs were declared safe. After demonizing the cholesterol in them for a generation, nutritionists finally acknowledged there was overwhelming scientific evidence that eggs were not artery-clogging killers after all.

But wait. The government’s latest nutrition guidelines came out this month, and they’re not egg-friendly. They say people should consume as little cholesterol as possible. That’s even stricter than the 2010 standard allowing 300 milligrams a day, about the amount in one egg.

Why would the same nutrition scientists who said last year that “cholesterol is not considered a nutrient of concern for overconsumption” keep warning people not to eat it?

The answer lies in some of the less-than-scientific beliefs held by nutritionists. Underlying their endeavor is the faith that there are good foods and bad foods – and that by strictly avoiding the bad foods, we can conquer heart disease and cancer, and perhaps put off death itself.

That faith has led them to warn people away from anything that presents even the remotest possibility of causing harm. It’s a misuse of the precautionary principle: the idea that substances should be treated as dangerous until scientifically proven safe.

Food choices can certainly influence health. There’s a strong consensus that too much sugar is a risk factor for obesity and diabetes, for example. But too much caution can do more harm than good. Applying the precautionary principle to food fails to take account of alternatives. When told not to eat one thing, we reach for something else.

Provisional evidence that butter and cream caused heart attacks led to increased consumption of margarine and non-dairy creamer instead. Many heart attacks and bypass operations later, research determined that the trans fats in these substances were much worse.

Trans fats – aka hydrogenated vegetable oils – are manufactured through a process that renders them chemically distinct from the fats coming from plants and animals. For much of the 20th century, they were a major component of margarine as well as commercial pastries, processed foods and snacks. The stuff not only raises bad cholesterol but also lowers good cholesterol and boosts triglycerides.

The health strictures against eggs went along with a general demonization of fats. So for years people ate more carbohydrates – a prescription that many experts now admit played a role in the current epidemic of obesity and type 2 diabetes.

Meanwhile, there was never good evidence that eggs had more than a minor effect on blood cholesterol or that eating them in moderation was harmful.

Some people develop abnormally high blood cholesterol because the mechanism for cleaning up the excess gets broken. The biggest risk factors for inadequate cleanup are genes, trans fats and, to a lesser extent, saturated fats. Not eggs.

Why can’t the guidelines reflect this? The U.S. Department of Agriculture’s explanation is that foods high in cholesterol also have lots of saturated fat. But that’s misleading. Eggs have very little saturated fat.

Oh, and about those saturated fats found in meat, poultry, cheese and butter – the kind the French eat while remaining quite healthy: Their deadly reputation may be exaggerated.

The scientific literature is full of contradictory claims. A 2013 meta-analysis concluded that cutting back on saturated fat didn’t help prevent heart disease. Some nutritionists say that study was misleading because people were substituting carbohydrates for saturated fat. (Who could possibly have led them to do that?) The new guidelines tell people to replace the saturated fats with unsaturated fats – the stuff found in vegetable oils.

Much of the science of saturated-fat risk does not come from experiments. Instead, it’s based on observational studies that rely on self-reporting, which is notoriously unreliable. Steve Nissen, head of cardiology at the Cleveland Clinic, said he doesn’t believe science knows yet whether saturated fats belong on the bad list and unsaturated fats on the good. Other experts agree.

The reaction of many nutritionists was to say that the USDA didn’t make its recommendations scary enough. They blamed the food industry. (The egg lobby must have been out on a company picnic.) But if the nutritionists had their precautionary way, we’d all be subsisting on kale salad. With no cheese – and no assurance of living better or longer.

— The Washington Post News Service with Bloomberg News

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